Nonenzymic Browning Mainly Due to

According to the scheme, ascorbic acid is readily converted into pentoses, but this does not explain the differences in behaviour between ascorbic acid and pentoses. 2,3-Diketogulonic acid (and the corresponding enol) is very unstable and readily develops brown coloration even at low temperatures.550 It can also undergo oxidative fission. 

At pH 7.0 and 37 °C, the degradation of ascorbic acid continues further, the main products being threose, glyceraldehyde, xylosone, and 3-deoxyxylosone.551 Threose is more reactive compared with an aldopentose or an aldohexose. At pH 7.0 and 37 °C, it has a half-life of about 3.5 d. It seems probable that threose is a major factor in Maillard reactions involving ascorbic acid. 

AA and aspartame are often present simultaneously in soft drinks. When heated to 90 °C, the AA is oxidised to DHAA and condenses with aspartame to form the /V-2-pyi on-3-yl derivative (cf. ref. 557), which is strongly bitter and causes an off-flavour.560 On storing solutions containing AA + aspartame and DHAA + aspartame (0.5% of each) at 5 and 37 °C for up to 150 d, the pyrone was formed in both solutions at 37 °C, but only in the latter at 5 °C. Intensity of bitterness correlated nearly linearly with browning. The highest amount of pyrone ( 1 g I. ) had been formed in the latter solution at 37 °C at 60 d. 

Impairment of AA homeostasis in diabetic humans and animals seems to be linked to the pathogenesis of diabetic complications, but AA degradation is difficult to disentangle from that of sugars. Nishikawa et al.561 have therefore developed a novel technique to follow AA catabolism, based on 6-deoxy-6-fluoroascorbic acid (FAA) and 19F-NMR spectroscopy, usually without the need for chromatographic separation. FAA was injected into normal and STZ-diabetic rats, whose plasma levels of FAA subsequently reached 42 and 27 fiM, respectively, implying accelerated cellular uptake in diabetics due to tissue depletion or accelerated oxidation and elimination in the urine. The urine contained 12-15 fluoro-substituted degradation products 

In STZ-diabetic rat livers, the levels of mRNA of 1-gulonolactone oxidase, catalase, and glutathione peroxidase were decreased at 6 weeks, as well as that of plasma alpha 1 proteinase inhibitor 3.562 AA synthesis enzyme and recycling enzyme mRNAs were also decreased, as was the level of AA itself. It seems that the antioxidative defence system had been severely damaged. 

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